RFR: 8369420: Parallel: Use multiple workers for class unloading [v3]

[Albert Mingkun Yang]

> Support parallel class unloading in ParallelGC. Much inspiration taken from `G1ParallelCleaningTask`.
> 
> Test: tier1-5

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27689/files
  - new: https://git.openjdk.org/jdk/pull/27689/files/93597b44..4ab4e65d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27689&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27689&range=01-02

  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/27689.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27689/head:pull/27689

PR: https://git.openjdk.org/jdk/pull/27689